At present, downhole pressure measuring systems utilized in a well bore include two general types, i.e. a Surface Recording Operation (SRO) type and a downhole Memory Recording Operation (MRO) type.
In an SRO, the downhole tool includes a downhole pressure/temperature sensing system which is powered by a surface located d.c. power source through an electric two conductor cable. The pressure.backslash.temperature data are measured in a downhole system and the data is transmitted to and displayed or recorded at the surface. The problem with the SRO is that the cablehead connecting the cable to the well tool electrically fails (because of well fluid leakage and contamination) on a frequent basis so that the tool malfunctions or shuts down and does not recover data during the shut-down. Pressure/temperature tests in a well bore can extend from one day to several weeks and a malfunction can result in serious delay, interruption and expense in obtaining the desired data.
In an MRO, the downhole tool is pressure sealed in an enclosure and is powered by d.c. batteries. The pressure/temperature data is collected in a memory in the tool over a period of time. Subsequently when the tool is recovered to the surface, the data is read out of the memory at that time. A system for data read out is disclosed in U.S. Pat. No. 4,763,259, issued Aug. 09, 1988 and in U.S. Ser. No. 08/020,393, filed Feb. 22, 1993. Since the tool is suspended in the well bore with a non-electrical slick line, if the tool malfunctions while in the well bore, or fails to operate properly there is no surface indication of the malfunction, and the malfunction will not be discovered until the tool is retrieved after the period of testing.
Because of the forgoing problems some operators have utilized two separate tools, an SRO tool and an MRO tool in tandem where an MRO tool is merely attached to the lower end of a SRO tool. If both tools are working, data is recovered at the surface with the SRO and also recovered downhole in the memory of the MRO. If the SRO fails, the operator then hopes, but does not know, if the MRO was functioning before, at or after the time that the SRO fails. This approach to overcoming deficiencies of a SRO tool requires the use and expense of two separate tools which results in a long tool and requires the expense of excess equipment including two pressure sensors.